Traditionally, pointing devices for use with computer systems, or other systems with a graphical user interface, have been in the form of a computer mouse. These devices have become so widespread that computer users have become familiar with the manner in which they are used in conjunction with a graphical user interface.
Optical pointing devices have been developed to overcome a number of shortcomings associated with a more traditional mechanical computer mouse. A mechanical computer mouse has a housing on its bottom surface in which a rubber coated steel ball is situated. When the mouse is moved in a particular direction across a surface the ball rolls in the same direction. Inside the housing there are two rollers, or wheels, spring loaded such that they are always in contact with the surface of the ball. One roller is positioned to roll only in a forwards and backwards direction, relative to the center of the mouse, and one in a left to right direction, relative to the center of the mouse. The rollers convert the movement of the mouse into electrical signals representing forward/backward movement and left/right movement. Usually, the conversion from mechanical movement of the rollers to electrical signals is effected by having a shaft encoder on each roller. A light source is placed on one side of the shaft encoder and a photodetector on the other. When the shaft encoder is rotated by the roller the emitted light from the source passes through the slits of the shaft encoder, effectively pulsing light to the photodetector and therefore giving a digital representation of the movement of the mouse.
Since this conversion to electrical signals is mechanical, a number of problems arise. Firstly, lint and dirt, over time, are picked up by the rubber ball and deposited on the rollers and inside the body of the mouse. This impinges on the performance of the mouse to the extent that a roller will stop turning. Although the rubber ball may be removed and the rollers cleaned, this is usually a tricky and unpleasant task. Second, the surface of the rubber ball or the surface of a mouse pad may become worn or damaged. This can lead to inconsistent movement of the onscreen pointer and can lead to frustration during operation of the mouse.
Optical pointing devices were designed to overcome these shortcomings. A modern optical mouse has no moving parts for conversion of movement to electrical signals and requires no special surface to operate on. In fact, apart from smooth glass, optical mice can operate on virtually any flat surface.
Generally, an optical mouse has a infrared (IR) light emitting diode (LED) light source positioned at a low angle of incidence with the flat surface on which the mouse is sitting. The surface below, when illuminated at this low angle of incidence, provides many physical features which are communicated in the reflected IR light and focused onto a pixel array in the mouse. Each pixel is then digitized and together this information provides a snapshot or frame of the surface below.
The pixel array is read out at regular intervals producing successive frames. Each frame is compared to the frame immediately previous and the direction of movement can then be inferred. To ensure that a good comparison can be made, the rate at which frames are read must be high enough so that features captured in a previous frame are still present in the current frame, although not necessarily in the same position.
The LED may be pulsed on and off to control the exposure for altering the average amount of light which reaches the array of photodetectors. Pulsing the LEDs also has the benefit of reducing power consumption as the LED is not always turned on and drawing power.
Power consumption is an important factor generally in optical pointing devices, but especially in wireless optical pointing devices. A wireless optical pointing device is usually powered by batteries, and therefore the power consumed by the device directly relates to the length of time that the batteries will last before requiring recharging or replacing.